Field of the Invention
This invention pertains to composite structures of ceramic bodies coated or joined together by ceramic eutectic compositions, and to a method for forming such structures, and more particularly pertains to such structures including ceramic eutectic compositions in the alumina, slica, rare earth oxide systems.
It is common practice to coat ceramic materials with various other materials to give smooth, chemical resistant and shock resistant surfaces. The principles of glazing have been used for centuries and are adequately discussed in most ceramic texts. See for example: Elements of Ceramics, F. H. Norton, Addison-Wesley Publishing Company, Reading, Massachusetts, 1957; and Introduction to Ceramics, W. D. Kingery, John Wiley & Sons, Incorporated, New York, New York, 1967. Usually glazes are applied to a completed ceramic piece as a slurry of blended finely ground oxides and salts and then fired to form the coating at temperatures somewhat below the usual densificaton temperature of the ceramic material to be coated. In some instances, it is possible to densify the green ceramic and glaze in a single operation.
Such operations require high temperatures and careful control of conditions to avoid formation of occulusions or inclusions along the weld line, which would weaken the structure.
In many operations, especially in the preparation of phosphors for television tubes and fluorescent lighting, it would be quite advantageous to have a glaze to apply to silica, silicon carbide or other ceramic materials, in order to minimize contamination of the phosphors from the ceramic containers of these materials used in firing. Such glazes would have to be quite chemically resistant to attach by the fluxing agents often used in preparation of the phosphors. Such glazes would also have to be capable of repeated and strenuous thermal cycling. Until the present time the only glazes or coatings which have been found to be effective have been vapor deposited films of materials such as yttrium slicate, which are relatively expensive to produce. Thus, for economic reasons these coatings do not answer the problem of providing easily applied coatings possessing high thermal shock resistance and high chemical resistance to attack.
The joining of ceramics is very common in instances such as mortaring of brick, block, etc. It is usually not necessary to form a strong weld or joint between ceramic pieces and little information is available on such formation. Where strong joints are necessary between similar ceramics, they are usually made by direct welding or fusing the pieces together, as is commonly done in the joining of fused silica tubes. However, it is unusual to attempt to join dissimilar ceramic materials by this technique, due to differences in thermal expansion.
There are instances in which some ceramic materials possessing desirable characteristics for certain applications are nevertheless economically or physically unsuited for use in thick slabs or massive pieces. It would therefore be advantageous if such materials could be applied as thin films or layers and tightly bonded to a more economical or more readily machinable base ceramic. For example, yttrium silicate, an expensive ceramic with excellent thermal shock and chemical attack resistance cannot be bonded to an inexpensive base ceramic such as silica in a manner to withstand high temperatures except by cumbersome and expensive techniques, such as vapor deposition.